The present invention relates to rare-element metallurgy; more specifically, it is directed to processes for extraction of gallium from sodium aluminate liquors.
The process of this invention is sure to find wide application in alumina production. The valuable specific properties of gallium account for its soaring popularity in nuclear power engineering, semi-conductor instrument making and rocket engineering.
All currently known techniques of gallium extraction from sodium aluminate liquors depend on the type of process for the production of sodium aluminate liquors which, in turn, depends on the grade of the alumina-containing material.
The Bayer technique of bauxite processing consists, first, in decomposing the feedstock with sodium hydroxide solutions (recycle sodium aluminate liquors), with most of the gallium passing into the sodium aluminate liquors. The latter decompose, and, the aluminum hydroxide precipitate having been separated, most of the gallium remains in the mother liquor. The mother liquor is partially evaporated to become a recycle liquor which is directed to leach a fresh batch of bauxite.
The mother and recycle liquors obtained in the Bayer process serve as feedstock in gallium production.
The so-called "red sludge" produced in the decomposition of bauxite with a sodium hydroxide liquor contains some 0.002% gallium by weight. In order to reduce the losses of gallium, the "red sludge" is subjected to sintering, the sintering mother liquor produced thereby likwise presenting a potential source of gallium.
It is known in the art to extract gallium from the sodium aluminate liquors produced in the Bayer process by electrolysis in mercury cathode cells.
Essentially, this technique consists in the following: the feedstock sodium aluminate liquor is decomposed and then subjected to electrolysis on a mercury cathode and a nickel anode at a temperature of from 40.degree. to 70.degree. C., a cathode current density of from 0.35 to 0.45 amp/sq.in., an anode current density of from 10 to 12 amp/sq.in., and a cathode potential of 1.9 v. In the course of electrolysis, gallium is reduced on the cathode and diffused into the metal mass with stirring. The electrolysis goes on for 24 hours to produce a 0.5-1.0 percent gallium amalgam, for the solubility of gallium in mercury at a temperature of 50.degree. C. is 1.5% by weight. The amalgam is decomposed with a sodium hydroxide solution by heating to the point of boiling. The level of gallium in the sodium hydroxide solution reaches 30 to 80 g/lit. This solution is subjected to gallium electrolysis in a vessel with a solid steel (or nickel) cathode or a liquid gallium cathode. The resultant metallic gallium usually contains a large amount of zinc, lead, iron, nickel and copper impurities.
The gallium current yield in the mercury cathode cell never exceeds 5.35% by weight, since most of the current is used up for impurity reduction and hydrogen evolution.
So, it takes up to 3 tons of mercury to produce 1 kilogram of gallium according to the technique described. In order to cut down on the mercury consumption, Landi from Italy developed an improved electrolysis celll incorporating a rotary cathode formed as a hollow drum coated on the external surface thereof with a thin layer of mercury.
At a cathode current density of 0.45 amp/sq. dm., 0.04 kg of gallium can be produced on one square meter of the drum surface within 24 hours, the gallium current yield being 2.8 percent and the power consumption rate being 155 Kwt.hr. per 1 kg of gallium (see: "Aluminio" Journal, 1959, No. 5, pp. 219-24, Italy).
The latter known method has some serious disadvantages:
the electrolysis cell with the rotary mercury cathode is undesirably sophisticated;
the mercury consumption rate is still excessively high, 2 kg per 1 kg of gallium;
a lot of mercury is wasted in sludge; and
the most serious disadvantage of the process is its very low gallium current yield.
As compared with the latter process, the method of producing gallium by cementation with the aid of a sodium amalgam is more efficient.
The cementation process is more effective than direct electrolysis on a mercury cathode, too.
However, both of these two types of processes have serious drawbacks:
the mercury vapors are toxic;
a high proportion of mercury is lost in the sludge;
the probability of contamination of the sodium aluminate liquors with mercury is high;
the low solubility of gallium in mercury requires large quantities of mercury to be processed; and
finally, the mercury electrolysis and sodium amalgam cementation methods are actually nothing else than techniques for gallium concentration which must be followed by at least another two operations, viz. (a) separation of the gallium from the amalgam; and (b) final electrolysis of the concentrated gallate solutions to produce metallic gallium.
It is likewise known in the art to produce gallium by electrolysis on flat steel cathodes from gallate solutions obtained by dissolving a gallium concentrate.
According to literary data, at a cathode current density of 0.07 amp/sq.m. and an electrolysis time of 5.5 days, the degree of gallium extraction from the solution averages 86.7% by weight and the residual concentration of gallium in the solution is 0.336 g/lit.
The solid electrodes also show considerable drawbacks: passivation of the electrode surface requiring longer electrolysis times; contamination of gallium with the electrode material; and the difficulties involved in attempts to automate the process.
Gallium electrolysis on a liquid gallium cathode is more effective than the solid cathode process. Gallium electrolysis from pure sodium gallate solutions on a liquid gallium cathode and a nickel anode shows a current efficiency of from 40 to 50 percent (see, for example, G.D.R. Pat. No. 27,024.).
The liquid gallium cathode method offers several advantages over the solid cathode technique which stem from the fact that the liquid cathode is easier to agitate, thereby eliminating concentration limitations and minimizing cathode passivation; this also enables the process to be rigorously conducted at a predetermined set of optimal conditions since the electrode surface usually remains constant. However, this technique, too, has a disadvantage which consists in the need to preconcentrate gallium so as to bring its level in the electrolyte to 2 or 3 g/lit, while it is common knowledge that gallium concentration is an ineffective operation involving considerable process losses.
It is also currently known in the art to employ a process for the extraction of gallium from the sodium aluminate liquors produced in the Bayer process by precipitation with finely distributed (finely comminuted) aluminum (see, for example, U.S. Pat. No. 3,170,875). According to this method, aluminum is to be added in a quantity sufficient for the formation of a gallium-aluminum alloy wherefrom the gallium is recovered chemically or electrochemically. However, the use of finely divided aluminum to extract gallium involves a high rate of consumption of the cementing agent, viz. aluminum. Another disadvantage of the latter known method consists in the need to follow the cementation operation with the decomposition of the product gallium-aluminum alloy to recover the gallium thereof.